The mechanical properties of alloy steels vary with the properties of their free metal boundaries, grain bodies and grain and phase boundaries. Current practices rely on many alloying systems and thermomechanical treatments, such as rolling, pressing, hammering and forging and various chemical and heat treatments to alter the mechanical properties of alloy steels. Current alloying systems are based on the idea of steel microstructure modifications and do not consider the effects of grain boundaries between crystals and alloy phase components on mechanical properties.
Iron (Fe), carbon (C), manganese (Mn), phosphorous (P), sulphur (S), silicon (Si), and traces of oxygen (O), nitrogen (N), and aluminum (Al) are always present in steel, together with alloying elements, such as nickel (Ni), chromium (Cr), copper (Cu), molybdenum (Mo), tungsten (W), cobalt (Co) and vanadium (V). Current alloying systems, steel making and heat treatment practices often produce non-equilibrium segregations of traditionally harmful admixtures (S, P, Sn, etc.) as well as embrittling non-metallic phases on free metal surfaces, grain and phase boundaries during tempering. Chemical heat treatments, such as nitro-carburizing and nitriding cause brittleness and distortion of grain bodies due to formation of a second, large volume phase along grain boundaries, having a harmful effect on the viscous characteristics of steel. For example, the impact strength of steel containing (by weight) 0.25% C; 1.6% Cr; 1.5% Ni; 1.0% W; and 0.6% Mo, is reduced to 2-3 J/cm.sup.2, following oil quenching at 980.degree. C. and a 24 hour temper at 500.degree. C. (false nitriding).
Another aspect of current steel alloying, making and heat treatment practices is that increases in strength decrease ductility, and in the alternative, increases in ductility decrease strength. Heretofore, no satisfactory compromise has been found between strength and ductility of alloy steels.
Current practices require large numbers of classes and grades of alloy steels, large investments and large inventories to support the requirements of industrial and consumer products. More than 320 grades of specialty steels are produced in the United States; 70-100 in Germany; 140-160 in Great Britain; 60-70 in Sweden; 140-160 in France; 100-120 in Japan; and 140-150 in Russia.
The following alloying systems are typical of current practices:
A: Structural, heat-treatable, carburizing, nitro-carburizing, and nitriding steels PA1 B. Die, spring, maraging, and duplex steels PA1 C. High speed tool steels PA1 D. High temperature steels PA1 E. Free-cutting steels
1. Fe--C--Cr PA2 2. Fe--C--Cr--Mo--Al PA2 3. Fe--C--Cr--Ni--Mo PA2 1. Fe--C--Cr--Si PA2 2. Fe--C--Cr--Si--V--B PA2 3. Fe--C--Cr--Si--Ni--Mo--(V, Ti)--N PA2 1. Fe--C--Cr--W--Mo--V--Co PA2 1. Fe--C--Cr--Ni--Mo--Si--(V, Ti, Nb) PA2 1. Fe--C--Cr--(Ca, Pb, Se, Te, Sb)
Another aspect of the current practice is that vast, complex facilities are required to support the many current alloying systems. Large sums of money are required to establish and maintain large inventories and complex facilities.